This project consists of a series of tissue culture studies on the neurobiological role of the gonadal steroid hormones (estrogens and androgens) in the differentiation and development of steroid receptor-containing regions of the developing brain. These experiments are designed to elucidate further some cellular, metabolic and molecular aspects underlying the organizational effects of the gonadal steroids during brain development. These studies will focus on certain fundamental questions concerning steroidal effects; notably: (i) their role in the differentiation and development of steroid receptors and receptor-containing cells; (ii) the phenotype and neural sited of steroid targets during development; and (iii) whether steroid effects are mediated directly or indirectly by interactions with growth factors (insulinrelated) endogenous to the brain. Morphological observations of living cultures of the fetal and newborn brain will be correlated with (1) a wide variety of histological procedures including silver stains, immunocytochemistry for phenotypic markers of neurons and neuroglia and for insulin-related peptides, and 125I-estrogen autoradiography; (2) nuclear assays of estrogen receptor content; (3) immunobinding assays for a membrane surface glycoprotein (5B4) associated with neurite growth; (4) recombinant DNA technology (Northern blots, dot blots, in situ hybridization) to determine estrogen regulation of genes associated with neurite growth (B-tubulin and neurofilament (KD 200) protein and with the insulin-related peptides; and (5) metabolic studies of steroid metabolism (aromatase activity). These multidisciplinary studies represent a novel and unique approach to an understanding of developmental mechanisms underlying the genesis of sexually differentiated behaviors and neuroendocrine functions about which there is still very little known. These studies are of critical significance to an understanding, as well, of neural plasticity during normal development, including the differentiation of cognitive functions in the human, and are related to the origins of a wide variety of developmental abnormalities in the human which are of considerable clinical, socio-cultural and educational importance. These include certain types of reproductive infertility, the postulated sexually dimorphic disorders of cognition, and problems of psychosexual differentiation.